A thin film transistor (hereinafter, referred to acronymously as a “TFT”) has been widely used as a switching device for display of, for example, a liquid crystal display device. The cross-sectional structure of a typical TFT is shown in FIG. 11. The TFT of FIG. 11 has a gate electrode, an insulator layer, and an organic semiconductor layer formed on a substrate in this order, and has a source electrode and a drain electrode formed with a predetermined distance therebetween on the organic semiconductor layer. In the thus structured TFT, the organic semiconductor layer serves as a channel region, so that an electric current flowing between the source electrode and the drain electrode is controlled by a voltage applied to the gate electrode, and, as a result, the TFT performs an on-off operation.
Conventionally, this TFT has been made of amorphous or polycrystalline silicon. However, a conventional problem resides in the fact that a CVD apparatus used to manufacture TFTs using such silicon is highly costly, and hence a great increase in manufacturing costs is caused when a display device using TFTs or a similar device is made large in size. Another conventional problem resides in the fact that a process for making a film of amorphous or polycrystalline silicon is performed at a very high temperature, and therefore limitations are imposed on the kind of material usable as a substrate, and therefore, for example, a resin substrate cannot be used although the resin substrate is light in weight.
To solve these problems, a TFT that uses organic substances instead of amorphous or polycrystalline silicon has been proposed. A vacuum deposition method, an application method, etc., are known as film-forming methods employed when TFTs are formed of organic substances, and, according to these film-forming methods, the device can be made large in size while curbing an increase in manufacturing costs, and a process temperature required during a film-forming process can be made comparatively low. Therefore, a TFT that uses organic substances has the advantage of being small in limitations imposed when material to be used for a substrate is selected, and this TFT is expected to come into practical use. Examples of TFTs using these organic substances are shown in, for example, Non-Patent Literatures 1-4 in a list below. Examples of organic substances used for an organic-compound layer of a TFT include a multimeric complex, such as conjugated polymer or thiophene (see Patent Literature 1 shown below), and a condensed aromatic hydrocarbon, such as pentacene (see Patent Literature 2 shown below), which are p-type substances. Examples of organic substances used for an n-type field-effect transistor (i.e., n-type FET) include 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA), 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TCNNQD), and 1,4,5,8-naphthalene tetracarboxylic diimide (NTCDI), which are disclosed by Patent Literature 3 shown below.
On the other hand, an organic electroluminescence (EL) device is known as a device that uses electric conduction in the same way. The organic EL device is generally structured so that an intense electric field of 105 V/cm or more is applied in the direction of the film thickness of an ultra-thin film of 100 nm or less so as to allow an electric charge to forcibly flow, whereas the organic TFT is required to allow an electric charge to flow at high speed in an electric field of 105 V/cm or less over a distance of a few microns (μm) or more, and hence is required to allow its own organic compound to become more conductive. However, organic compounds used for conventional organic TFTs are small in field-effect mobility, are low in operation speed, and have the conventional problem of being inferior in high-speed responsiveness that is a necessary characteristic of a transistor. Still another conventional problem is that the on/off ratio of the transistor is small. The term “on/off ratio” mentioned here denotes a value obtained by dividing an electric current flowing between a source and a drain when a gate voltage is applied (i.e., in an ON state) by an electric current flowing between the source and the drain when a gate voltage is not applied (i.e., in an OFF state). Ordinarily, the “ON-state current” denotes a current value (i.e., saturation current) obtained when a gate voltage is gradually increased, and, as a result, an electric current flowing between the source and the drain is saturated.